Hard-disk drives have rotating high-precision disks that are coated on both sides with a special thin film medium designed to store information in the form of magnetic patterns. Electromagnetic read/write heads suspended or floating only fractions of micro inches above the disk are used to either record information onto the thin film medium, or read information from the medium.
A read/write head may write information to the disk by creating an electromagnetic field to orient a cluster of magnetic grains in one direction or the other. Each grain will be a magnetic dipole pointing in a certain direction and also creating a magnetic field around the grain. All of the grains in a magnetic region typically point in the same direction so that the magnetic region as a whole has an associated magnetic field. The read/write head writes regions of positive and negative magnetic polarity, and the timing of the boundaries between regions of opposite polarity (referred to as “magnetic transitions”) is used to encode the data. To increase the capacity of disk drives, manufacturers are continually striving to reduce the size of the grains.
The ability of individual magnetic grains to be magnetized in one direction or the other, however, poses problems where grains are extremely small. The superparamagnetic effect results when the product of a grain's volume (V) and its anisotropy energy (Ku) fall below a certain value such that the magnetization of that grain may flip spontaneously due to thermal excitations. Where this occurs, data stored on the disk is corrupted. Thus, while it is desirable to make smaller grains to support higher density recording with less noise, grain miniaturization is inherently limited by the superparamagnetic effect.
In response to this problem, engineers have developed patterned media, where the magnetic thin film layer is created as an ordered array of highly uniform pillars, each pillar capable of storing an individual bit. Each bit may be one grain, or several exchange-coupled grains, rather than a collection of random decoupled grains. In this manner, patterned media effectively reduces noise by imposing sharp magnetic transitions at well-defined pre-patterned positions, known as bit patterns. Bit patterns are organized as concentric data tracks around a disk.
One benefit of patterned media is the ability to overcome the above described superparamagnetic effect. Due to their physical separation and reduced magnetic coupling to one another, the magnetic pillars function as individual magnetic units, comprised either of single grains or a collection of strongly-coupled grains within each pillar. Since these magnetic pillars are typically larger than the individual grains in conventional media, their magnetization is thermally stable.
Conceptually, patterned media is a simple notion; however, mass producing disks at a reasonable cost is an immense challenge. Fabrication of a patterned media disk requires, in one example, etching a pattern of pillars or islands in the surface of a disk, depositing a non-magnetic material on the etched surface of the disk (which subsequently covers the islands), and “planarizing” the surface of the disk until the magnetic islands are again uniformly exposed. However, chemical-mechanical planarization of patterned disks currently requires significant over-polish to achieve uniformity. The over-polish can damage the inside and outside diameter of the disk.